Adding math kerns (aka cut-ins or staircase kerning) to OpenType math fonts

Question

Is it possible to add math kerns (aka cut-ins or staircase kerning) to OpenType math fonts? If yes, how?

I am personally looking for something which works for LuaLaTeX (using fonts.handlers.otf.addfeature?), but I would be happy for other solutions as well.

For example, I want to add a bottom right math kern to \lmathccal{O}.

\documentclass{article}

\usepackage[newcmbb]{fontsetup}

% Below code copied from here: https://tex.stackexchange.com/a/373539/128462.

\DeclareFontFamily{U}{mathc}{}
\DeclareFontShape{U}{mathc}{m}{it}%
{<->s*[1.03] mathc10}{}
\DeclareMathAlphabet{\lmathccal}{U}{mathc}{m}{it}
% \DeclareMathAlphabet\cmmathcal{OMS}{cmsy}{m}{n}
% \renewcommand{\symcal}{\lmathccal}

\begin{document}
    \(\lmathccal{O}_i\) \(\symcal{O}_i\)
\end{document}

(1) you should not use ulrike's answer with a unicode maths setup. U changes the encoding so it is no longer unicode. (2) kerning pairs, as far as I know, are always intra-font so would not apply unless you created a combination virtual font. — cfr
– cfr, Commented 5 hours ago

mickep · Accepted Answer · 2025-11-30 20:23:30Z

Too long for a comment, and it might or might not help you (you have to test), but in the ConTeXt testsuite there is a file kerns-002.tex (here with a line typing the file out in the pdf removed):

\startluacode

    local kerns = {
        ["TeXGyrePagellaMath-Regular"] = {
            [0x1D449] = { bottomright = { { kern = -200 } } }, -- math italic V
            [0x1D44A] = { bottomright = { { kern = -100 } } }, -- math italic W
        }
    }

    local function addkerns(tfmdata)
        if tfmdata.mathparameters then
            local kerns = kerns[tfmdata.properties.fontname]
            if kerns then
                local characters = tfmdata.characters
                for unicode, specification in pairs(kerns) do
                    local character = characters[unicode]
                    if character and not character.mathkerns then
                        character.mathkerns = specification
                    end
                end
            end
        end
    end

    fonts.handlers.otf.features.register {
        name         = "mathkerns",
        description  = "additional math kerns",
        default      = true,
        initializers = {
            position = 1,
            base     = addkerns,
            node     = addkerns,
        }
    }

\stopluacode

\setupbodyfont[pagella] \showglyphs

\starttext

    Don't use this method in \ConTeXt\ as we have
    other mechanisms for it!

    \startformula V^i_j = W^i_j \stopformula

\stoptext

As you see in the end there is a warning to ConTeXt users not to do this. But it has an effect also in ConTeXt (with luametatex and luatex). With luatex the relevant part looks like this:

Compare with the same file, without the lua snipped enabled:

Good luck!

actually, maybe the op is trying to use opentype. I always forget what fontsetup does ... but then switching to U encoding looks very wrong? edit: no, my apologies. I think they are using MATH, but are then kind of ignoring that fact ... — cfr
– cfr, Commented 5 hours ago

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